The present invention relates to upgrading coal by removing sodium ions. More particularly, the invention relates to removing sodium ions from coal by means of water-washing and ion-exchanging.
The presence of large amounts of sodium in coal is undesirable as it contributes to fouling of combustion facilities. The fouling problems can arise if sodium exceeds about 3%w (as Na.sub.2 O), yet several important deposits of coal contain considerably more than that much sodium. Thus, a process which can economically remove, for example, 30-70% of the sodium, can be very desirable and can be a prerequisite condition for exploitation of sizeable deposits.
The levels of sodium oxide in the ash at which coal combustion can lead to fouling problems are not yet clearly defined, and can be different for different coals. Nevertheless, levels in excess of about 3%w are not desirable and coals with more than 4%w in the ash, are generally difficult to market. Some Powder River Basin coal samples have been found to yield over 6%w of sodium oxide. And, coals with sodium oxide contents in excess of 8 or 9%w have also been found and documented.
U.S. Department of Energy publication DOE/GFETY/RI-82/3 "Conceptual Design of a 1.6 MM Tons/Year Lignite Preparation Facility for Sodium Reduction" describes ion exchanges of coal with 0.11 normal sulfuric acid for various times and solid to liquid ratios and indicates that a sodium oxide content decreased from 8.5 to 0.99% and the selectivity of the ion removal varied with variations in residence time.
A publication from Pennsylvania State University Fuel Science Program "Ion Exchange in Selected Low Rank Coals, Part I Equilibrium, Part II Kinetics", Solvent Extraction, Ion Exchange, 1:4, 813-825 (1983), describes measurements of equilibrium ion exchange behavior for metal cations with hydrogen ions and indicates the exchange to be a linear function of pH regardless of the cation concentration of the solution; with the extent of exchange being a function of available hydrogen ions.